As will be understood by those skilled in the art, engineered particleboards including oriented strand boards or "OSB" boards are less expensive than solid wood boards and wood laminates, strong and light in weight. However, such particleboards are limited in their application and use by the construction industry, for example, because engineered particleboards are strongly hydrophilic. When exposed to water or moisture, for a period time, the average moisture content of OSB boards, for example, increases to about twenty percent. When manufactured, however, OSB boards have a moisture content of less than about two to three percent. Thus, particleboards such as OSB boards generally cannot be used for such applications as building foundation forms, roof sheathing, basement walls, etc. OSB boards for example include relatively large wood particles, strands or chips in a resin matrix or binder. The exposed surfaces of engineered particleboards are also relatively rough and include spaces or interstices between the particles which must be filled if the exposed surfaces are coated with a smooth waterproof coating, particularly where the coating is relatively thin, such as a coating having a thickness of about 0.020 inches.
Engineered particleboards, such as OSB boards, comprise particles of wood or wood-based products, such as paper, in a resin matrix or binder, wherein the particles are oriented to provide strength and rigidity to the boards. Generally, the preferred binder is methylene bis phenylisocyanate or methylene diphenyl isocyanate (MDI) which is applied to the particles or wood chips by spraying the liquid binder into a blender. In the manufacture of oriented strandboards, for example, debarked logs are cut into strands or wafers which are dried with heat and screened to grade for strands of the correct size. The dried strands are then coated with a binder such as MDI and transported in layers on a conveyor to a forming line, where the layers are cross-oriented into mats. For face layers, the strands generally run along the panel while the core layers are randomly oriented or run across the panel. The mats are trimmed to a workable size and then transported to a press where the wood strands and glue are bonded under heat and pressure to create a structural panel.
Engineered particleboards such as oriented strand boards may be covered, for example, by adhesively bonding a plastic sheet over the surfaces of the boards; however, this method is relatively expensive and thus eliminates the cost advantage of such boards. The applicants have also determined that oriented strand boards for example may be coated directly with a polyurea coating. However, the polyurea coating must be relatively thick (greater than about 0.040 inches) and polyurea coating polymers are relatively expensive. Further, it was determined by the applicants that conventional methods may not be used to coat particleboards such as oriented strand boards with polyurethane because of the high moisture content of OSB boards and because the polyurethane coated surface has numerous surface imperfections including pin holes, bubbles, etc. Further, the relatively large wood particles and the interstices between the particles will "read" through the polyurethane coating. That is, the exposed surface of a relatively thin polyurethane coating having a thickness, for example, of 0.020 inches will conform to the wood particles and interstices between the particles making the polyurethane coated boards unacceptable for certain applications. As will be understood by those skilled in the art, the hydroxyl component of polyurethane will react with water and therefore it is generally not possible to coat a panel having a relatively high moisture content of greater than about five percent.
Thus, there has been a longfelt need for a relatively inexpensive method of coating engineered particleboards such as oriented strand boards, particularly a coating that is waterproof and relatively smooth. It is also important in certain applications such as building foundation forms to use panels or boards which are relatively free of surface imperfections, such that the surface imperfections are not imprinted on the poured foundation. Pin holes which expose the particleboards to moisture must also be avoided in roof sheathing, for example, and other applications where the boards or panels are exposed to moisture. Therefore, such applications are now limited to solid wood panels or laminates which are less hydrophilic. The method of forming a polyurethane coating on engineered particleboards such as oriented strand boards of this invention solves the above problems by providing a thin smooth waterproof coating which is relatively inexpensive, particularly when compared to other polymer coatings such as polyurea. Further, the polyurethane coating method of this invention may be utilized as a final step in the manufacturer of engineered particleboards or may be used to coat such boards at a separate facility.